Embodiments of the present disclosure generally relate to data lock detection systems and methods that are configured to determine whether a data signal, such as non-return-to-zero (NRZ) data signal, is locked or unlocked with a clock signal.
Non-return-to-zero (NRZ) is a form of digital data transmission in which binary low and high states are transmitted by specific and constant direct current voltages. In positive-logic NRZ, the low state may be represented by the more negative or less positive voltage, and the high state may be represented by the less negative or more positive voltage. For example, a logic state of 0 may be represented by 0.5 volts, while the logic state of 1 may be represented by represented by 1 volt. Conversely, in negative-logic NRZ, the low state may be represented by the more positive or less negative voltage, while the high state may be represented by the less positive or more negative voltage. For example, a logic state of 0 may be represented by 1 volt, while the logic state of 1 may be represented by 0.5 volts. In general, NRZ data includes binary code in which 1s are represented by one significant condition (such as a positive voltage) and 0s are represented by another significant condition (usually a negative voltage), with no other neutral or rest condition.
NRZ data may be displayed with respect to an eye diagram or pattern. In an eye diagram, a data signal is repetitively sampled and applied to a vertical input, while the data rate is used to trigger a horizontal sweep. The display is referred to as an eye diagram because the pattern appears as a series of eyes between a pair of rails.
An eye diagram provides an indication of the quality of signals in high-speed digital transmissions, for example. An oscilloscope, for example, may generate an eye diagram by overlaying sweeps of different segments of a long data stream driven by a master clock. When many such transitions have been overlaid, positive and negative pulses are superimposed on each other. When many bits are overlaid, the resulting image appears similar to the opening of an eye.
Typically, it is desirable to determine the phase relationship between a sample clock and NRZ data. The phase relationship between the sample clock and the NRZ data is defined to be the same (that is, there is no difference between the phase of the sample clock and the NRZ data) when the data is being sampled at the center of the eye diagram. Data sampled at the center of the eye is typically reliable. However, when the phase relationship between the sample clock and the NRZ data differs, data may no longer be sampled from the eye center, and therefore the data may be less reliable. Accordingly, determining whether the NRZ data is locked with respect to the sample clock provides information as to the reliability of the data.
A known system and method for determining whether NRZ data is locked with a sample clock includes a reference oscillator with quadrature phases. However, such a system and method may be relatively complex and may require a relatively large amount of energy to operate.